Overview of recent ALICE results

Transcription

1 Overview of recent ALICE results E. Scomparin (INFN-Torino) A short introduction Heavy ions at the LHC The ALICE experiment Characterization of the QGP The physics program Global observables The most liquid liquid that ever existed Light flavors and strangeness The chemistry of the QGP Hard probes Transport properties of the QGP p-pb vs Pb-Pb a reference or more? Open points and prospects The future of ALICE 1

2 Heavy ions at the LHC Investigate the region of the phase diagram of strongly interacting matter corresponding to the highest possible temperature and the lowest possible net baryon density Critical point Re-create the first (and hottest!) liquid that ever existed and that gave rise to matter around us. and study its properties in the laboratory! 2

3 The energy frontier Evolution of (some) properties of the system with the collision energy (N.B. approximate values!) Central collisions SPS RHIC LHC s (GeV) (today) dn ch /d ( =0) Energy density (GeV/fm 3 ) V (fm 3 ) (from HBT) Decoupling time (fm/c) (from HBT) Average QGP temperature (MeV) (photons, dileptons) LHC hotter, larger and longer lived fireball! 3

4 A complex (and long!) endeavour (Almost) exactly 30 years ago: first heavy-ion beams at CERN! From the press highlights of that time 4

5 A complex (and long!) endeavour (Almost) exactly 30 years ago: first heavy-ion beams at CERN! From the press highlights of that time 5

6 A complex (and long!) endeavour (Almost) exactly 30 years ago: first heavy-ion beams at CERN! From the press highlights of that time 6

7 A complex (and long!) endeavour (Almost) exactly 30 years ago: first heavy-ion beams at CERN! From the press highlights of that time 7

8 ALICE: a complex (and long!) project March 1992 Evian meeting "Towards the LHC Experimental Programme" March 1993 The ALICE Letter of Intent Use L3 magnet TPC-based tracking Emphasis on PID At that time Only y~0 No muon-dedicated detectors 42 institutes, 230 people 8

9 ALICE nowadays A world-wide Collaboration Goal exploit the unique physics potential of nucleus-nucleus interactions at LHC energies 9

10 The ALICE detector General-purpose detector for the study of QGP-related signals at the LHC, with several unique features (see next slides) 10

11 ALICE performance Excellent PID capabilities down to very low p ~ 100 MeV/c ITS Track impact parameter resolution allows for reconstruction of secondary vertices from D decays TPC 11

12 ALICE performance TRD TOF HMPID TOF, HMPID significantly extend PID capabilities TRD allows triggering on electrons in the central barrel 12

13 ALICE performance (hard probes) Jet physics via EMCAL (trigger) Muon triggering (forward-y) allows for detection of heavy quarkonia (J/, (2S), (1S,2S,..)) Full coverage of QGP-related signals from low to high-p T, at both central and forward-y 13

14 The collision systems Pb-Pb collisions Hot matter effects Soft + hard probes p-pb collisions Calibrate cold nuclear matter effects (CNM) pp collisions Reference for Pb-Pb studies, QCD (mainly soft) Change of paradigm at LHC energies! High-multiplicity p-pb and pp collisions show intriguing signals of QGP-like effects 14

15 Collision centrality Nuclei are extended objects Geometry related to observables via Glauber Model Related to multiplicity or forward energy (spectators) 15

16 The SPS/RHIC inheritance SPS/RHIC results crucial steps in our understanding of QGP SPS evidence for deconfinement signals NA57, J.Phys.G 32 (2006) 427 NA50, PLB477(2000) 28 Clear signal of J/ suppression beyond CNM effects Clear enhancement of strange hyperon production 16

17 The SPS/RHIC inheritance Two major discoveries at RHIC STAR, PRL 86 (2001) 402 PHENIX, PRL 88 (2002) Strong elliptic flow (close to hydro limit) Quenching of high-p T particles in central Au-Au collisions QGP as a perfect liquid, opaque to hard probes traversing it 17

18 ALICE data samples run 1 System s NN (TeV) L int Year pp nb pp nb pp pb pp pb p-pb nb Pb-Pb nb Event recording bandwidth: 1.25 GB/s for Pb-Pb events (dominated by TPC R/O) Overall data (MC, raw and reconstructed) on permanent storage Tape: ~45 PB Disk: ~55 PB 18

19 ALICE data samples run 2 System s NN (TeV) L int Year pp pb Pb-Pb nb pp nb p-pb nb p-pb 8.16 ~20 nb New detectors installed Complete TRD Install DCal Extended PHOS Install CPV in front of PHOS 19

20 Pb-Pb collisions 20

21 Pb-Pb collisions 21

22 Pb-Pb collisions QUARKONIA PHOTONS AND E.M. PROBES FLOW AND CORRELATIONS QGP JETS AND HIGH-p T HADRONS HEAVY QUARKS 22

23 Charged particle multiplicity Steep rise in (dn ch /d )/0.5 N part when moving from RHIC energy to LHC (from logarithmic at low energy to power law) Possibly related to the significant increase of the contribution of hard processes Interesting test for theoretical calculations sensitive to the modelling of the initial state of the collision (gluon saturation) ALICE, PRL 105, (2010) 23

24 Charged particle multiplicity run 2 ALICE, PRL 116 (2016) % increase for the most central collisions with respect to s NN =2.76 TeV, in agreement with the previously established power-law dependence indicates a similar increase in the energy density reached 24

25 Centrality dependence of (dn ch /d ) ALICE, PRL 116 (2016) Factor ~1.8 on charged multiplicity per participant pair, from peripheral to central collisions Shape of the centrality dependence remarkably similar to RHIC Models tuned at s NN =2.76 TeV reasonably reproduce the 5.02 TeV data 25

26 Transverse energy and energy density Use tracking detectors and PHOS/EMCAL ALICE, PRC 94 (2016) Results consistent with CMS in 10-80%, (slightly) lower in 0-10% Shape of centrality dependence is similar at RHIC and LHC Assuming a formation time 0 =1fm/c Energy density Bj = GeV/fm 3 (0-5%) In the core of the reaction region Bj =21 2 GeV/fm 3 To be compared with critical energy density Bj ~ 1 GeV/fm 3! 26

27 Direct photons and temperature Direct photon spectra at p T >5GeV/c are in agreement with pqcd pp calculations scaled by N coll Central Pb-Pb collisions (0-20%): 2.6σ excess for 0.9< p T <2.1 GeV/c Low p T dominated by thermal photons T eff = 304±11±40 MeV (~30% higher than RHIC) Interpretation of the inverse slope parameter not trivial A correlation with the initial temperature exists Two contributions blue-shifted photons from the late stages (high radial flow) high-t photons from early stages ALICE, PLB 754 (2016)

28 Azimuthal anisotropy Impact parameter + beam direction reaction plane Correlation between azimuthal emission angles and reaction plane Evidence for collective behaviour Non-central heavy-ion collisions geometrical anisotropy of the fireball In a hydro-dynamic scenario Azimuthal anisotropy mainly related to different pressure gradients in the collectively expanding medium 28

29 Azimuthal anisotropy Fourier expansion of the azimuthal distributions v 2 : elliptic flow: strongly related to the thermalisation of the medium v 3, v 4, : information on the geometric fluctuations of the initial state (odd harmonics should be zero w/o fluctuations) 29

30 Elliptic flow run 1 results Integrated elliptic low increases by ~30% from RHIC to LHC energy ALICE, PRL 105, (2010) At fixed p T, the elliptic flow values are very similar at RHIC and LHC energies higher integrated v 2 at LHC related to the p T increase 30

31 Higher flow harmonics Comparisons with hydro models: v 2 and v 3 values consistent with values of the viscosity to entropy density ratio close to the ideal fluid limit /s = 1/4 (Ultra) central events: v 2 strongly decreases, reaction region has a nearly spherical symmetry. Higher harmonics, dominated by fluctuations, are sizeable Also 2-particle correlations studies show evidence for anisotropies up to the 5 th harmonics ALICE, PRL 107 (2011)

32 Elliptic flow run 2 results ALICE, PRL 116 (2016) v n for the p T range 0.2<p T <5.0 GeV/c vs centrality (ratios are between 5.02 TeV and 2.76 TeV results) Average increase of (3.0±0.6)% for v 2, (4.3±1.4)% for v 3 and (10.2±3.8)% for v 4 from 2.76 to 5.02 TeV, for 0-50% centrality Results compatible with predictions from hydrodynamic models 32

33 Identified particles: flow ALICE PID capabilities allow detailed studies of flow harmonics for various particle species ALICE, JHEP 06 (2015) 190 Mass ordering can be interpreted in terms of an interplay between collective radial expansion and anisotropic flow At high p T, particles tend to group according to mesons vs baryons (but no precise n q scaling visible) Effects well reproduced by QGP+hadronic cascade models (VISHNU) 33

34 Identified particles: spectra Wide p T range accessible Sensitive to various physics mechanisms 1) Low p T : collective effects (radial flow) 2) Intermediate p T : fragmentation vs recombination 3) High p T : parton energy loss 34

35 Low p T : radial flow parameters Collective radial flow modifies p T spectra (Low-p T ) slopes are modified according to the particle mass and expansion velocity Blast-wave fits Extract T kin and T Central Pb-Pb collisions ALICE, PRC 88, (2013) ALICE, PLB 728 (2014) Kinetic freeze-out T kin <100 MeV Expansion velocity ~ 0.65c 35

36 Intermediate p T : baryon/meson ratio ALICE, PRL 111 (2013) Enhancement at intermediate p T Hydrodynamics describes only the rise at < 2 GeV/c Recombination (pushes baryons to higher p T ) reproduces effect but overestimates EPOS gives good description of the data (with flow) ALICE, PRC 91 (2015) p and have similar masses p/ flat vs p T in central Pb-Pb Mass determines the spectral shapes (as in hydrodynamics) 36

37 Moving towards high p T (hard probes) Particle ID available up to p T ~20 GeV/c! R AA = N Coll dn P AA dn P pp R AA <1 suppression R AA >1 enhancement High p T strong suppression with respect to N coll scaling NO sizeable difference vs particle species hadronization is not affected by the medium, the effect is due to parton energy loss in hot matter 37

38 Hard probes of the QGP Recent results on high p T hadrons Strong suppression up to p T =40 GeV/c Remarkable similarity between results at s NN =2.76 and 5 TeV Compensation between increasing suppression and modification of the shape of p T spectra? (Further) constraints to energy loss models 38

39 Moving to a detailed understanding Heavy quark R AA Energy loss expected to depend On parton color charge (g vs q) On parton mass (heavy vs light q) E g > E u,d,s > E c > E b leading to an R AA hierarchy R AA (B) > R AA (D) clearly observed (at low p T ) ALICE, JHEP 11 (2015) 205 CMS-PAS-HIN R AA (D) ~ R AA ( ) color charge dependency of energy loss compensated by the softer fragmentation and p T spectrum of gluons 39

40 Jets: more info on energy loss Measure energy loss distributions Longitudinal fragm. functions Transverse Jet profiles R AA =1 Strong jet suppression observed Evidence for significant out-of-cone radiation Similar R AA as in high-p T hadrons (caution on the comparison of the p T scale!) 40

41 Recent results on jet R AA Pb-Pb s NN =5.02 TeV Strong out-of-cone jet radiation Similar effect at s NN =5.02 and 2.76 TeV Denser medium smaller R AA Harder collisions larger R AA Compensation of the two effects Jet suppression reasonably well reproduced by energy loss MC models ALICE, PLB 746 (2015) 1 41

42 Heavy quarkonium Screening of strong interactions in a QGP Charmonium supppression c c c c T. Matsui and H. Satz, PLB178 (1986) 416 Perturbative Vacuum Color Screening Central AA collisions SPS 20 GeV RHIC 200 GeV LHC 2.76 TeV Large charm quark multiplicity at LHC energy may lead to a recombination mechanism which enhances charmonium production LHC 5 TeV N ccbar /event ~0.2 ~10 ~85 ~115 42

43 Charmonium suppression J/ψ is strongly suppressed in Pb-Pb collisions, as expected in case of color screening, but less at the LHC than at RHIC R AA does not vary with centrality for N part >100 Less suppression at low p T, contrary to RHIC Re-generation mechanisms are needed to describe data ALICE, JHEP 05 (2016)

44 Recent results on J/ 5.02TeV Similar suppression at s NN =2.76 and 5.02 TeV More accurate recent data confirm the remarkably flat behavior of R AA vs N part 2.76TeV ALICE, arxiv: R AA increases at low p T, at both energies, as expected in a regeneration scenario Hint for an increase of R AA, at 5.02TeV, in 2<p T <6 GeV/c 44

45 Recent bottomonium results Tendency to LESS suppression for the (1S) when increasing energy? R AA at s NN =2.76 and 5 TeV compatible, but also the y-shape but reminds recombination patterns (unexpected because of the relatively low b-quark cross section) Possible tension in the comparison with theoretical models 45

46 p-pb collisions 46

47 p-pb collisions LEOPARD TORTOISE ELEPHANT SHREW SMALL 5 RHINO BEETLE ANTLION BUFFALO WEAVER 47

48 Moving to smaller systems: pa Common wisdom use pa collisions to calibrate the size of cold nuclear matter effects and isolate QGP-related signals Example: R AA vs R pa for charged hadrons Demonstrates that the suppression signal observed at high p T in Pb-Pb collisions is NOT related to cold nuclear matter effects What about other hard probes? ALICE, EPJC 74 (2014)

49 pa results: jets ALICE, EPJC 76 (2016) 271 No effects on jets from CNM, even as a function of the centrality of the p-pb collision the suppression effect seen in Pb-Pb is due to hot matter effects 49

50 pa results: heavy quark production At both forward-y (muons from semileptonic heavy quark decays) and central-y (D-meson hadronic decays) no significant CNM effects What about soft/global observables? ALICE, PRL 113 (2014)

51 pa results: strangeness Saturation of strangeness production observed in Pb-Pb collisions for kaons and hyperons p-pb yields (normalized to pions) gradually reach the Pb-Pb value when increasing the event centrality Even in pp collisions, a similar increasing trend can be seen considering increasing charged particle multiplicities Contrary to hard probes, these results show typical hot matter effects also for small collision systems! ALICE, arxiv:

52 pa results: baryon/meson Clear evolution with multiplicity Mid-p T : ratio increases Low-p T : corresponding depletion Reminiscent of Pb-Pb phenomenology...generally understood (for Pb-Pb!) in terms of collective flow recombination Quantitatively similar when comparing event classes with similar N ch 52

53 pa results: v 2 ALICE, PLB 726 (2013) 164 P.Bozek et al., PRL 111(2013) Similar mass ordering observed for v 2 from two-particle correlations in p-pb Hydrodynamic models can describe the data Very surprising result! p-pb was supposed to be the reference system Applicability of hydrodynamics questionable Are we observing a pressure driven effect? 53

54 ALICE upgrade Conditions expected for LHC run 3 ( ) Pb-Pb peak interaction rate: 50 khz (now 8 khz) Present ALICE readout rate 1 khz Physics goal for run 3: High precision measurements of rare signals (focus on low p T ), to be reached through Increase of the readout rate to 50 khz Improvement of the pointing resolution at central (new ITS) and forward y (Muon Forward Tracker) The ALICE upgrade requires major improvements for the TPC and other detectors higher R/O rate Online reduction of the data volume, L int >10 nb -1 (100 times run 1) Technical Design Reports approved Moving to construction phase New ITS: 7 pixel 54layers 10 m 2 of Si 12.5 Gpixel

55 Conclusions (1) ALICE has collected a wealth of data through LHC run 1 and is now enriching its data sample in run 2 (p-pb run being completed now!) Very large set of observables investigated, from global variables to identified particle spectra, electromagnetic and hard probes (jets, high-p T hadrons, open heavy quark production, quarkonia, ) From the study of the hottest lump of matter created up to now at particle accelerators Confirm effects seen at SPS/RHIC extending them to a new energy scale Bring new discoveries, and among them Jets are strongly affected by the medium Mass-dependence of heavy quark energy loss Evidence for charmonium production through re-combination of deconfined charm quarks Strong bottomonium suppression Hydrodynamic description holds not only for large QGP volumes, but also for the medium created in smaller systems 55

56 Conclusions (2) LHC run 2 goes on at full speed No qualitatively different effects are expected moving from s NN =2.76 to 5.02 TeV, but the quality of the results is improved, thanks to higher luminosities and better understanding of the apparatus Recent ALICE run 2 results include Study of hadron multiplicity Elliptic flow Suppression of high-p T particles Charmonium and bottomonium Many more to come stay tuned! Physics program extending into run 3 and run 4 thanks to the substantial upgrades foreseen for LS2 56

57 p-pb, s NN =8.16 TeV Thank you! 57

58 Other stuff 58

59 Future of LHC heavy-ion program (today) 2016: p-pb run, shared between s NN = 5 TeV and s NN = 8 TeV EYETS: CMS pixel upgrade (for pp luminosity) 2018: Pb-Pb run, maximum available energy, L= cm -2 s -1 LS2: ALICE upgrades apparatus (TPC, ITS, MFT) stand 50 khz event rate expected for run-3 and improve tracking LHCb upgrades tracker higher granularity, push towards central collisions ATLAS new muon small wheel reduce fake trigger CMS muon upgrade add GEM for p T resolution, RPC for reducing background (better time resolution), extend coverage to > : LHC run-3, experiments require L int >10 nb -1 for Pb-Pb (compared to L int ~ 1 nb -1 for run-2) Possibility of accelerating lighter ions under discussion : LHC run-4 59

60 Quarkonia in 2016: from color screening Screening of strong interactions in a QGP c c c c T. Matsui and H. Satz, PLB178 (1986) 416 Perturbative Vacuum Color Screening Screening stronger at high T D maximum size of a bound state, decreases when T increases Different states, different sizes Resonance melting QGP thermometer 60

61 to regeneration At sufficiently high energy, the cc pair multiplicity becomes large Central AA collisions SPS 20 GeV RHIC 200 GeV LHC 2.76TeV N ccbar /event ~0.2 ~10 ~85 Statistical approach: Charmonium fully melted in QGP Charmonium produced, together with all other hadrons, at chemical freeze-out, according to statistical weights Kinetic recombination: Continuous dissociation/regeneration over QGP lifetime P. Braun-Munzinger and J. Stachel, PLB490 (2000) 196 Thews, Schroedter and Rafelski, PRC (2001) Contrary to the color screening scenario this mechanism can lead to a charmonium enhancement 61

62 CNM effects are not negligible! p-pb collisions, s NN =5.02 TeV, R ppb vs p T backward-y mid-y forward-y p-going Pb-going ALICE ALICE, JHEP 1506 (2015) 055 Fair agreement with models (shadowing/cgc + energy loss) (Rough) extrapolation of CNM effects to Pb-Pb R PbPb cold =R ppb R Pbp Evidence for hot matter effects! 62

63 (2S) in p-pb collisions (2S) suppression is stronger than the J/ one at RHIC and LHC shadowing and energy loss, almost identical for J/ and (2S), do not account for the different suppression Only QGP+hadron resonance gas (Rapp) or comovers (Ferreiro) models describe the strong (2S) suppression at LHC Accurate Pb-Pb results still missing! ALICE, JHEP 1412(2014)073, LHCb-CONF PHENIX, PRL 111 (2013)

64 Weak CNM effects for bottomonium R ppb close to 1 and with no significant dependence on y, p T and centrality Fair agreement ALICE vs LHCb (within large uncertainties) ALICE, PLB 740 (2015) 105 ATLAS-CONF LHCb, JHEP 07(2014)094 64

65 Anisotropic transverse flow In collisions with b 0 (non central) the fireball has a geometric anisotropy, with the overlap region being an ellipsoid Macroscopically (hydrodynamic description) The pressure gradients, i.e. the forces pushing the particles are anisotropic ( -dependent), and larger in the x-z plane -dependent velocity anisotropic azimuthal distribution of particles Microscopically Reaction plane Interactions between produced particles (if strong enough!) can convert the initial geometric anisotropy in an anisotropy in the momentum distributions of particles, which can be measured

66 Anisotropic transverse flow Starting from the azimuthal distributions of the produced particles with respect to the reaction plane RP, one can use a Fourier decomposition and write dn d( RP ) N 0 1 2v cos( ) 2 cos RP v RP... The terms in sin( - RP ) are not present since the particle distributions need to be symmetric with respect to RP The coefficients of the various harmonics describe the deviations with respect to an isotropic distribution From the properties of Fourier s series one has v n cos n RP 66

67 Other recent results on anisotropies More complex observables can be defined, which are more robust against systematic biases originating from non-flow effect E-by-E fluctuations of amplitudes of anisotropic flow harmonics ALICE, PRL 117 (2016) No single centrality where a single parametrization of /s describes BOTH correlations Central collisions very sensitive to various parameterizations of the MC- Glauber initial conditions 67

68 E-by-E fluctuations of amplitudes of anisotropic flow harmonics Symmetric 2-harmonic 4-particle Cumulant SC(m,n) with m n indicates averaging in two steps a) Over all distinct particle quadruplets in the events b) Single event averages weighted with `number of combinations SC(m,n) is zero if no flow fluctuations or if magnitudes of v m and v n are uncorrelated 68

69 Analyzing jet shapes p T -weighted width of the jet (low g for collimated jets) Dispersion of constituents in jets Fewer constituents higher p T D Core of jets in Pb-Pb appears to be more collimated than in pp Shift to higher p T D in Pb-Pb fewer jet constituents and larger p T dispersion 69

70 Identified particles: production rates Particle yields of light flavour hadrons described by thermal model fits Fit parameters T ch chemical freeze-out temperature B baryochemical potential Yields span 7 orders of magnitude Agreement within 20% (exception K *0 and p ) Chemical freeze-out temperature T ch ~ 156 MeV Hadrons are produced in chemical equilibrium in Pb-Pb collisions at the LHC, as at the SPS and RHIC 70

71 Heavy quark/quarkonium v 2 ALICE results show evidence for D-meson elliptic flow and strong indications for J/ v 2 in Pb-Pb collisions ALICE, PRC 90 (2014) D 0 J/ ALICE, PRL 111 (2013) Low (intermediate) p T non-zero v 2 indication for c-quark thermalization and further confirmation for J/ production by recombination 71